This invention relates to a method of counting random pulsed data and, more particularly, to such a counting method which is used to count random pulsed data that is subject to noise and pulse pile-up.
The frequency of a periodic signal is typically measured by counting the number of events (pulses, zero crossings, etc.) that occur over some fixed sample period, T. If C events are counted over this interval, the measured frequency of the signal, F, is simply calculated from the equation: EQU F=C/T
The rate of change in frequency can be calculated from two successive measurements of frequency, F.sub.1 and F.sub.2, from the relationship: EQU Rate=(F.sub.1 -F.sub.2)/t
The calculations of frequency (level) and rate of change of frequency (rate) are only as accurate as their inputs. The accuracy of the inputs is significantly affected by the randomness of the data, the noise present in the data, and pulse pile-up errors. The randomness factor is very significant at low count levels, noise interference occurs over the entire range of input frequency, and pulse pile up is most significant at high frequencies since it occurs when pulses arrive too close together to be detected separately.
Within a nuclear reactor, neutron activity is used as an indication of the power being generated by the reactor. In general, power is proportional to neutron activity and an indication of the rate of change of power within the reactor is important for both control and safety systems. Instruments designed to measure neutron activity, called source range instruments, must cover the range of neutron activity from less than one pulse per second to millions of pulses per second and must meet stringent design criteria with respect to the measurement of frequency and the rate of change of frequency of neutron activity. Such design criteria specify accuracy, stability and time response specifications. In order to meet the specifications, the input count must be as accurate as possible.
In a neutron activity monitoring environment, there are two classes of noise that need to be eliminated. They are the low background noise and intermittent noise. The background noise typically results from background gamma radiation and the intermittent noise typically results from large electromagnetic interference.
For a pulse, which represents neutron activity, to be counted, it must be detected. In typical source range instruments, there is a minimum amount of time which must occur between successive pulses in order for distinct pulses to be counted. This amount of time is called the dead time. The dead time of an instrument is determined by the neutron detector and the associated electronics. It will also be influenced by the average width of the input pulses. When designing a source range instrument, dead time is obviously kept to a minimum. However, because of the random nature of radioactive decay, there is always some possibility that a pulse will be lost because it occurs too soon after the previous pulse or before the previous pulse ended. The resulting dead time losses can cause up to a 35% error in pulse count at higher frequencies.
It is therefore desirable to have a method of counting random pulsed data which compensates for noise and pulse pile-up which occurs in a neutron activity monitoring system.